The object of this invention is to reduce the sensitivity of an ultrasonic intrusion detection system to ambient noises and yet maintain the sensitivity to moving targets. Bells and escaping compressed air or steam can produce ultrasonic components which fall within the normal range of doppler frequencies and, hence, are confused for moving targets thus creating false alarms.
In my pending patent application, U.S. Ser. No. 455,260, filed on Mar. 27, 1974, I describe a method for reducing the sensitivity to random to and fro motion and for ambient noise which is symmetrical about the transmitting frequency. That system, however, is sensitive to interference which is unsymmetrical about the transmitting frequency since that is the characteristic being sought from a moving target. An unsymmetrical interference spectrum is for example often created by the harmonics of a telephone bell.